Elongated tubular parts have traditionally been fabricated by employing a tapered, elongated mold in conjunction with a matingly tapered, elongated mandrel. For example, Scott et al U.S. Pat. No. 3,442,738 discloses a method and apparatus for making fiberglass shafts, and specifically fiberglass golf shafts, which involves the feeding of fiberglass threads or slivers between a mold and a tapered mandrel. Beaver et al U.S. Pat. No. 3,896,206 discloses a method for forming and curing a fiber reinforced hollow epoxy shaft wherein the fiber reinforced shaft is initially formed in a tapered mandrel. The assembly is placed in a mold and the mandrel is forced into the mold to compact the fiber reinforced plastic shaft. Hogarth U.S. Pat. No. 3,974,012 discloses apparatus and method for forming tapered tubular shafts. Thermosetting resin sheet material is wound transversely about a core which is surrounded by a shell.
It is also known to use some sort of heat and/or pressure activated bonding material or agent to fuse the raw material together en route to forming the finished article. Some examples are the Scott '738 patent (resin), the Beaver '206 patent (plastic); and the Hogarth '012 patent (thermosetting resin), as well as U.S. Pat. Nos. 3,962,394 to Hall (liquid resin); 4,298,562 to Latty (elastomer or plastomer); 4,380,523 to Lind et al (thermoplastic polymer); 4,389,269 to Cooper et al (resin); 4,774,043 to Beckman (settable plastic); 5,028,464 to Shigetoh (epoxy resin or other thermosetting or thermoplastic resin); and 5,048,441 to Quigley (polymer matrix).
Another method of forming a tubular part involves the use of a braided sleeve which is pulled over a multidiameter tube and impregnated with a settable plastic, after which the plastic hardens to form an article such as a drive shaft for an automobile. Such a technique is disclosed in Beckman U.S. Pat. No. 4,774,043.
In the construction of composite materials generally, numerous different substrate materials have been combined with either one of two matrix materials to form the composite: either thermoplastic matrix material or thermoset matrix material. As is known in the art, the setting of thermoset resins is characterized by cross linking. Upon heating of the thermoset resin it hardens and solidifies. The cure process for thermoset resin is irreversible, i.e., once cross linking has taken place, the curing process is completed and the resin will no longer flow, at least without degrading.
With thermoplastics, application of heat softens the thermoplastic material and enables it to flow. Upon cooling down the material solidifies and sets. Thus, while the curing of thermosets is essentially an irreversible process, thermoplastics can be remelted and reconfigured many times.
Three specific processes which have been employed in the fabrication of composite materials utilizing the thermoset matrix material are resin transfer molding, pultrusion, and "B stage" forming. In resin transfer molding, the substrate material, for example in the form of braid, is surrounded by a mold, and foam material is placed on the interior of the braid substrate. A vacuum assist is used to pump thermoset resin into the braid, and upon heating and curing the foam expands thereby providing compressive force on the braid substrate and urging thermoset matrix against the outer mold.
Pultrusion, similar to extrusion, involves the passing of the yarn substrate material first through a thermoset resin bath. The yarn is then fed into a dye, which includes one or more heated zones. As the yarns exit the end of the dye, the yarns are pulled from the finished end. The process provides low void content (no air bubbles) and good fiber wetting in the bath.
In B stage forming, the substrate and thermoset matrix are first passed through a drying oven to initiate some of the cross linking process to give the material some stability. The substrate and matrix material, normally in sheet form, are laid up into the desired geometry or form, and then covered by a bag. A vacuum is then pulled on the bag while the sheet is autoclaved to effectuate full cure of the material. Tubular parts can also be formed from this process.
The use of thermoset materials as the matrix material in the composite, however, has a number of disadvantages. Thermoset materials are generally relatively expensive. Thermoset materials can also be difficult to handle, requiring layup by hand or expensive machines. In addition, thermoset materials are relatively brittle. Due to the nature of the one-way process, thermoset materials are not reformable. Thus, errors made during forming are not remediable, thus resulting in expensive scrap. Some thermoset materials are environmentally unfriendly in that they do not biodegrade and contain ozone depleting chemicals. Lastly, thermoset materials have a short shelf life. Often such materials must be stored in a refrigerator to arrest advancement of the curing process, as thermoset materials tend to progress to cure on an unrefrigerated shelf. Typically, the materials can be frozen for only about one year.
Thermoplastics, on the other hand, can be remelted and reconfigured. Thermoplastics are often much tougher than thermosets, and in addition have an infinite shelf life. Thermoplastics are also environmentally friendly in comparison to thermosets.
However, the processes traditionally used to form parts from thermosets are not adequate when employing thermoplastics, as these processes cannot provide high enough temperatures and pressures to form parts from thermoplastics. In addition, while substrates with thermoset matrix materials have high "tack", which greatly aids in handling and forming of parts made therefrom, substrate materials made of a thermoplastic matrix material have little tack, thus creating a need for a process which will take this into account. Since a substrate having a thermoplastic matrix generally has insufficient tack, a traditional textile process such as braiding, weaving, etc, is needed to handle the material. However, such a textile process creates what is known as a high bulk factor, which is the ratio of preform thickness to finished part thickness. This high bulk factor results in the fibers of the substrate material being out of plane which greatly reduces the strength and stiffness of parts made therefrom.
In summary, there is a need for a process to form composite materials generally and tubular parts specifically from a substrate material in combination with a thermoplastic matrix material, rather than a thermoset matrix, since thermoplastics have many more advantages than do thermosets. However, current molding processes utilized in the forming of parts from the combination of substrate with thermoset matrix cannot provide the higher pressures and higher temperatures required for, nor account for the lack of tack, high bulk and unusual geometries characteristic of, the fabrication of parts utilizing thermoplastic matrix material.